Everyone has their own opinion, and I've heard it said that any milk fat (butter, cream, etc) added to a risotto diminishes it's authentic quality. Of course, that depends on where you grew up and how you learned to make risotto.
But for me adding heavy cream, thus making risotto mantecato, is almost cheating. You're cheating to attain the heavenly creaminess. Where I much prefer using technique and good old fashioned mama's know-how. Stirring the heck out of the risotto when first adding the liquid releases the most starch from any stage of the cooking process.
I've actually never made a risotto with heavy cream before starting work at a new job. The chef doesn't dash a little into a pan, but pours in about a 1/2 cup or more for each serving. So if you are a heavy cream user I'd also be interested in knowing how much you typically use.